>How many Robinhood users are aware that when they open an account, transfer money, and buy stocks all in the same day they're actually buying on margin?

Maybe I'm out of touch but I would guess the answer is "most". Having to wait a couple days for money to clear after being transfered to a financial institution is extremely common.

When I first deposited on RH and saw that the money was available for trading immediately my first thought wasn't "Oh the money cleared instantly", rather it was "Wow, I'm surprised they're willing to take on that risk with new accounts". Turns out, maybe RH should have given that risk a little more thought.